This invention relates to a multimode beam forming network for circular array radar antennas which provides back fill-in for the antenna pattern.
The above mentioned cross referenced related patent application describes a beam forming network which has particular use in an air traffic management system. Briefly, aircraft are generally equipped with transponders which are periodically interrogated by local ground stations. More particularly, the ground station transmits a coded interrogation message along a narrow rotating beam into its sphere of interest. An aircraft illuminated by the ground station beam, as the beam is rotated about the ground station as the center, decodes the coded interrogation message and responds with the requested information such as aircraft identification or altitude depending on the exact format of the coded interrogation message. By considering the instantaneous antenna beam pointing angle the ground station determines the azimuth of the responding aircraft and by timing the round-trip interrogation/response cycle the ground station determines slant range to the responding aircraft. Of course, as mentioned above, the response will include responding aircraft identity and altitude so that the ground station will be able to determine the positions of aircraft traffic within its sphere of interest.
The coding scheme used by the ground station not only is intended to elicit a response from an illuminated aircraft but also to ensure with a relatively high degree of certainty that aircraft which are not within the narrow beam do not respond to the interrogation message which might also be carried on the narrow beam side lobes. This is accomplished by the ground station transmitting one portion of the coded interrogation message, known as a P2 pulse in the art, on an omnidirectional beam, and by transmitting the remainder of the interrogation message on the narrow beam. An aircraft illuminated by the narrow beam will thus perceive the P2 pulse as relatively lower in amplitude than the remainder of the interrogation message, while an aircraft outside the narrow beam will perceive the P2 pulse as relatively higher in amplitude than the remainder of the same message. Each transponder's decoder is equipped to discern this distinction and will cause the transponder to respond when the P2 pulse is perceived to be lower but will cause the transponder to be temporarily suppressed so it will not respond when the P2 pulse is perceived higher in amplitude. As might be expected, this is quite important since an aircraft which responds to what can be termed a side lobe interrogation, that is, an interrogation not intended to be responded to by that aircraft, will incorrectly be perceived by the ground station as being on the instantaneous pointing azimuth of the narrow beam which, of course, that particular responding aircraft is not.
The method and means for implementing the above mentioned coding scheme of the prior art has certain faults, one of the most serious of which is the inability of the prior art ground station system to ensure that all aircraft not within the narrow beam are positively suppressed. Because of the antenna pattern side lobes an aircraft outside the narrow beam but within the ground station sphere of interest may fail to "hear" the interrogation message which, if it had heard it, it would have perceived as having a relatively high P2 pulse and thus would have suppressed itself. Thus, although the aircraft will not respond to that particular interrogation message, which of course it should not since it is outside the narrow beam, neither will it be suppressed. Normally the aircraft would have its transponder suppressed, that is, it would not respond during a short predetermined suppression period even though it may be illuminated by the proper interrogation message. This interrogation message which the aircraft might receive during its suppression period might, for example, be transmitted from a secnd, further removed, ground station whose sphere of interest should not extend into the sphere of interest of the first mentioned ground station, but which because of atmospheric or siting problems now does. It can be seen that should the aircraft respond to the interrogation from the second ground station the first station will interpret the response erroneously, that is, it will interpret that response as being indicative of an aircraft in the pointing direction of its narrow beam which, in this case, the responding aircraft is not. The ground station will also make an incorrect determination of slant range to the responding aircraft since it has correlated the interrogation/response cycle incorrectly.
The above problem was effectively solved by the beam forming network described in the above mentioned related patent application which included a back fill-in network to provide an interrogation message antenna beam pattern with an essentially true omnidirectional antenna pattern outside the narrow beam of the interrogation message. Thus, all aircraft outside the narrow beam described in the above mentioned patent application "hear" essentially all interrogations, perceiving a relatively high P2 pulse, so that they are suppressed constantly when outside the narrow beam and thus will not respond inadvertently. The above mentioned beam forming network, and more particularly azimuth beam forming network was composed of a back fill-in network which was essentially an omnidirectional network, a sum pattern network, a low sidelobe difference pattern network and a network for combining the patterns generated by the other networks in order to produce the desired sum and difference antenna patterns.
More particularly, the beam forming network of the above mentioned patent included N output terminals, one of which was terminated by its characteristic impedance, at which the weights corresponding to the desired sum and difference antenna patterns were generated. This was done by generating N/2 signal weights corresponding to a sum antenna pattern at a sum pattern network, splitting each signal in equal halves and delivering the split signals, in phase, respectively to pairs of output terminals. Since one output terminal was terminated by its characteristic impedance the weight at its associated output terminal corresponded to an omnidirectional antenna pattern. The zero order mode terminal was chosen as this latter terminal. Thus the weighted signals corresponding to a sum antenna pattern having omnidirectional sidelobes were generated. The same scheme was used to obtain the difference pattern weights from a difference pattern network except that the N/2 signals corresponding to a difference antenna pattern were split so that one split signal was 180.degree. or nearly so out of phase with respect to the other split signal. In addition, power was coupled from the difference pattern network to the sum pattern network to provide weights corresponding to a cardioid-shaped antenna pattern which were superimposed on the difference pattern weights to generate weights corresponding to a difference antenna pattern having omnidirectional sidelobes. However, since during generation of the difference pattern weights signals were received at the output terminal corresponding to an omnidirectional antenna pattern from both the sum pattern network and the difference pattern network there was an undesirable skewing of the difference pattern weight from the desired 180.degree. condition. In addition, since all signal weights were split equally by signal splitters in the form of 3 dB hybrids, including the weight corresponding to the zero phase shift omnidirectional antenna pattern, an excessive amount of signal energy was lost at the terminated terminal.